Conventionally, photometry devices of this type include a microprocessor activated by a clock that is generated through a clock generating device. The light receiving element is activated by a clock signal generated within the microprocessor. A photometric signal obtained from the light receiving element is input to the microprocessor, and the brightness value of the subject is obtained through a method that will be described hereinafter.
First, the luminosity L on the light receiving element is calculated through formula 1, based on the output V of the light receiving element. EQU L=V/(S*t) (1)
where V is the output voltage (in units of volts) output from the light receiving element, S is the sensitivity value of the light receiving element (in units of volt/Lux*second), t is the accumulation time given to the light receiving element (in units of seconds), and L is the luminosity (in units of Lux) on the light receiving surface of the light receiving element.
The brightness value BV of the subject field is obtained from formula 2, using a correction coefficient Z (unitless) that is calculated based on the lens data obtained from a lens ROM, the luminosity L calculated by formula 1, and a correction coefficient FO for the beginning diaphragm value (in units of AV). EQU BV=(log L/log 2)+Z+FO (2)
where BV is the brightness value (in units of BV) of the subject field.
In addition, there is a method, shown by formula 3, wherein the subject field brightness value BV is directly calculated without calculating the luminosity L on the light receiving element. EQU BV=(log (V/t)/log 2)+Z'+FO. (3)
In this case, the correction coefficient Z', which takes into account the sensitivity value S of the light receiving element, is used in place of the correction coefficient Z.
FIGS. 10(a) and 10(b) graphically illustrate the operation of a light receiving element. In the three formulae above, when the graph of the output voltage V is extended in the t=0 direction, as shown in FIG. 10(a), the relationship between the accumulation time t and the output voltage V is assumed and set such that V=0 at t=0.
However, in actuality, an offset voltage V0 arises such that V is not equal to 0 at t=0. This is primarily due to the smear phenomenon.
The smear phenomenon is described in detail below. FIG. 5 illustrates an example of a light receiving element 9 that includes a CCD (i.e., charge-coupled device) area sensor. The portions indicated by the shaded regions are light receiving components (i.e., photoelectric conversion components), which are divided into a plurality of regions. A plurality of H registers (95H-1)-(95H-12) are connected to a single V register, 95V, and the photometric output is transmitted in series from the leading end of the V register 95V. The components other than the light receiving components on the light receiving element 9 (i.e., the components other than those indicated by the shaded regions) are shielded by a material, such as aluminum, to ensure that photoelectric conversion does not occur, even if light from the subject field reaches these components.
However, if a powerful light is incident upon the light receiving element, the light will pass through the shielding material and become photoelectrically converted. This will produce an increase of the apparent accumulation time and will cause the appearance of an offset voltage in the output.
Another phenomenon in which an offset voltage occurs will next be described. FIG. 6 is an enlarged view of the H register component of the light receiving element. In FIG. 6, reference number 91 designates a shutter drain, 92 designates a shutter gate, 93 designates a sensor, 94 designates a lead out gate, and 95H designates an H register.
FIGS. 7-9 diagrammatically illustrate a cross section taken along A-B in FIG. 6. As shown in FIG. 7, prior to accumulation, the shutter gate 92 of the light receiving element 9 is in an open condition (i.e., the low potential condition). The charge generated in the sensor 93 falls to the shutter drain 91 through the shutter gate 92.
Simultaneously with the commencement of the accumulation of the charges, the shutter gate 92 assumes the position shown in FIG. 8, and the charge generated in the sensor 93 begins to accumulate in the sensor 93. When accumulation is finished, as shown in FIG. 9, the readout gate 94 opens, and the charge is conducted to the H register 95H.
Limited time intervals are necessary in the actions of the shutter gate 92 and the leadout gate 94. During these time intervals, a charge is still being generated in the sensor 93. This charge is added to the charge generated during the accumulation time. This appears as the offset voltage.
Since the conventional photometry devices are not made for dealing with these types of phenomena, errors of the magnitude of the offset voltage are introduced into the photometry value.